A vehicle mounting a power train combining an internal combustion engine (by way of example, use of a known engine such as a gasoline engine or a diesel engine may be possible) and a rotating electric machine (motor generator or the like), referred to as a hybrid system, has been developed and put to practical use. In such a vehicle, output share of an operation by the internal combustion engine (hereinafter also referred to as an engine) and an operation by the rotating electric machine is automatically switched independent from an amount of accelerator operation by the driver, to be regulated to attain the highest efficiency. For example, when the engine is operated for rotating a generator operated in a steady state for charging a power storage device such as a secondary battery, or when the engine is operated intermittently during running in accordance with the amount of charge of the secondary battery, start and stop of the engine operation are repeated independent of the amount of accelerator operation by the driver. Specifically, by operating the engine and the electric motor separately or in a coordinated manner, mileage can be improved and emission of exhaust gas can significantly be reduced.
In this manner, the engine of a hybrid vehicle is intermittently driven during running and, therefore, stop control takes place frequently. In engine stop control, an approach may be taken in which rotational energy of the engine (kinetic energy) is applied to a generator and the energy is converted to electric energy by the generator for stopping the vehicle. The electric power generated at this time is used for charging the secondary battery.
Japanese Patent Laying-Open No. 10-306739 (hereinafter referred to as Patent Document 1) discloses a power output device including a prime mover (corresponding to an engine), a triaxial power input/output means and two electric motors, in which rotation speed of the prime mover is quickly set to zero when the operation of prime mover is stopped. The power output device is for outputting power to a driving shaft, and includes: a prime mover having an output shaft; a first electric motor having a rotation shaft and inputting/outputting power to/from the rotation shaft; a second electric motor inputting/outputting power to/from the driving shaft; triaxial power input/output means having three axes coupled to the driving shaft, the output shaft and the rotation shaft, respectively, and when power is input to/output from any two of the three axes, for inputting/outputting power determined by the input/output power to/from the remaining axis; fuel stop instructing means for instructing stop of fuel supply to the prime mover when conditions to stop operation of the prime motor are satisfied; and stop control executing means for adding torque to the output shaft and regulating deceleration of rotation of the output shaft to a prescribed range to stop the prime mover in accordance with the instruction to stop fuel supply to the prime mover.
In the power output device, when conditions to stop the operation of the prime mover are satisfied, the power output device instructs stop of fuel supply to the prime mover and executes stop control. In the stop control, torque in a direction opposite to the direction of rotation is applied to the output shaft of prime mover so that deceleration of the output shaft is limited in a prescribed range and the prime mover is stopped. As a result, deceleration of the output shaft is limited within a prescribed range and, by way of example, regulation to quickly pass through a torsional resonance range, becomes possible. At the same time, it becomes possible to avoid wasteful power consumption in the electric motor.
In the device described in Patent Document 1, by the engine stop control, the rotational energy of engine is converted to electric energy by the power generation operation of the rotating electric machine (first electric motor), and recovered. The electric energy is typically used for charging a battery.
However, if tolerable input power to the battery is limited, for example, if the state of charge (SOC) of the battery is almost full or if the battery is cold, the electric power generated during the engine stop control could be excessive, possibly resulting in excessive charging of the battery. If the engine stop control involving power generation operation by the rotating electric machine is executed in such a state, excessive charging may cause damage or decrease battery life.